Aqueous lubricant compositions have been known for many years and have been applied to a variety of technologies including metal cutting and forming, the lubrication of oil drilling equipment, etc. One important application is the lubrication of the interface between a container and a moving conveyor line or track surface. Many common conveyor lubricants are based on fatty acid formulations. Such fatty acids are natural products comprising commercially available cocoa or tallow acids. The use of alkyl amines, phosphate esters, .alpha.-olefin sulfonates and amphoteric materials such as imidazolines and amino carboxylic acids in formulated lubricants have also been attempted.
As is known to those skilled in the art to which the present invention pertains, there has been an increasing usage of P.E.T. containers for beverages and other foodstuffs. Such containers are normally filled by passing them through filling and capping stations controlled by conveyor systems.
To ensure proper operation of the filling and capping systems, it is vital that the conveyor systems be continuously lubricated. Without adequate lubrication, the containers may stack up along the conveyor system, impeding their movement.
Thus, the conveyors are continuously lubricated by applying a lubricant to the conveyor, such as by spraying or the like. Conventional lubricants contain fatty acids, nonionic surfactants, alcohols, potassium hydroxide and other constituents, which in various combinations have functional disadvantages. For example, fatty acid lubricants form insoluble calcium salts when diluted with hard service water. Conventional lubricants are often incompatible with plastic, e.g. P.E.T. containers disposed along the conveyor system, causing them to eventually crack in transit or storage. Indeed, it has long been known that exposure by such P.E.T. containers to incompatible lubricants leads to a phenomenon which has been identified as "stress crack failure."
The lubricants commonly used on the load-bearing surfaces of these conveyor systems, such as those used in the food processing, beverage and brewery industries, typically contain fatty acid soaps as the active lubricating ingredient, because of the superior lubricity provided by fatty acid soaps.
The fatty acid soaps are generally formed by neutralizing a fatty acid with a caustic compound such as alkali metal hydroxide (NaOH or KOH) or an alkanolamine (MEA, DEA or TEA) and have an alkaline pH. Fatty acid soaps neutralized with such caustic compounds are generally incompatible with polyethylene terephthalate to such an extent that prolonged contact frequently results in the formation of stress cracks and fissures in the plastic. This is most frequently observed in bottling plants where carbonated beverages are placed into polyethylene terephthalate bottles. The stress placed upon the bottle by the bottling process and the internal pressure of the carbonated beverage contained within the bottle can cause stress cracks and fissures.
Various polyethylene terephthalate-compatible lubricant compositions have been developed by replacing at least a portion of the fatty acid with other lubricating components. For example, Rossio, U.S. Pat. No. 4,929,375 suggests that incorporation of a tertiary amine such as a (C.sub.8-10) alkyl dimethyl amine into a fatty acid lubricant composition enhances the polyethylene terephthalate compatibility of the lubricant composition.
While these various attempts have been successful in producing lubricant compositions which are compatible with polyethylene terephthalate, such compositions have not generally been effective for providing both superior lubricity and superior compatibility with synthetic polymeric packaging materials.
Anderson et al., U.S. Pat. No. 4,521,321 teach conveyor track lubricant compositions employing a phosphate ester comprising an ethoxylated fatty alcohol phosphate ester in combination with a fatty amine oxide in an aqueous solution. The active ingredients are used at a concentration of about 100 to 200 ppm. Stanton et al., U.S. Pat. No. 4,604,220 teach an .alpha.-olefin based conveyor lubricant that can contain a minor amount of other ingredients including anionic phosphate esters. Scharf et al., U.S. Pat. No. 5,062,979 teach a soap-free conveyor lubricant comprising an alkoxy phosphate ester alkyl benzene sulfonate and a carboxylic acid. Rossio, U.S. Pat. No. 5,223,162 teaches a method for inhibiting stress cracking in a PET article which uses a hydrophilic substituted alkyl aryl anionic surfactant. One phosphate ester composition sold under the trademark TRITON.RTM.H-66 by Rohm and Haas Company is disclosed. Aepli et al., U.S. Pat. No. 3,860,521 disclose an aqueous lubricating concentrate for conveyor systems that comprises a fatty acid soap, a surfactant and a monostearyl phosphate. McDaniel, U.S. Pat. No. 5,001,114 teaches alkyl monoglycoside and polyglycoside phosphate esters and anionic derivatives thereof. Gutzmann, U.S. Pat. No. 5,352,376 teaches an aqueous lubricant composition containing an alkyl polyglycoside material in combination with organo phosphates including alkyl orthophosphate such as a stearyl (fatty alcohol) phosphate, an alkyl phosphate ester, etc. Despo, U.S. Pat. No. 5,391,308 teaches an alkaline aqueous lubricant concentrate containing a fatty acid, an alkyl phosphate ester and an alkyl aryl phosphate ester that operates both as an emulsifying agent and as a stress crack inhibitor.
A substantial need exists to develop active lubricant materials and methods that reduce or eliminate the presence of fatty acid ingredients, lower the pH of the lubricant solution, do not cause stress cracking in plastic, e.g. polyethylene terephthalate (P.E.T.) bottles and remains stable over a wide variation of pH. The common belief that alkalinity is a major cause of stress cracking has led to a customer preference for low alkalinity lubricants. In other instances the lubricant is not stable over a wide pH range. Consequently, the present invention solves a different combination of problems than the prior art compositions, allows use of the lubricant over a wide pH range, and prevents or inhibits stress cracking in P.E.T. containers.